Pluto, Mars water and more! The biggest space stories of 2015

NASA's Dawn spacecraft captured this amazing of Ceres in the asteroid belt, showing the dwarf planet's Occator Crater in false-color. Dawn arrived in orbit around Ceres in March 2015.
(NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Pluto and its largest moon Charon as seen in natural color by NASA's New Horizons spacecraft during its historic July 2015 flyby of the dwarf planet.
(NASA/JUAPL/SwRI)

Dark, narrow streaks on Martian slopes, like these at football field-length features at Hale Crater on Mars are signs of seasonal flows of liquid water. NASA scientists announced the discovery in September 2015.
(NASA/JPL-Caltech/Univ. of Arizona)

The year that was 2015 swept in a significant amount of space and astronomy science.

NASA's New Horizons probe reached Pluto after a decade of travel across interplanetary space, exploring the most well-known dwarf planet. But this wasn't the only great solar system story this year; Ceres, Mars and the moons of Saturn also revealed their secrets. Exoplanets abounded, and discoveries swept in from the far reaches of the universe.

Hello, Pluto!

NASA's New Horizons made history in July as it zoomed by Pluto, passing within 7,800 miles (12,500 kilometers) of the dwarf planet's icy surface. The mission revealed a world with a large, heart-shaped feature unofficially known as the Tombaugh Regio, named for Pluto discoverer Clyde Tombaugh. [See the latest Pluto photos by New Horizons]

Even more amazing than the dwarf planet's famous heart, however, was the surprising fact that Pluto appeared to be geologically active, despite scientific predictions that the frigid world would have frozen into inactivity long ago. The Pluto images revealed a young surface with few impacts, indicating that the surface of the planet was reshaped by something — most likely ice — fairly recently in its history. The world boasts snakeskin-like terrain that bears no resemblance to the features of other worlds in the solar system.

New Horizons allowed scientists to finally nail down the size of the planet. The probe also revealed a completely round world with no bulge at its equator, suggesting the planet has or recently had a subsurface ocean beneath its crust.

Pluto's moon Charon revealed a few surprises of its own. Although not as young as its companion, Charon has a relatively young surface that seems to have undergone similar resurfacing processes. The northern pole boasts a strange reddish tint, most likely acquired from Pluto's captured atmosphere. While the moon has fewer mountains than its planet, one of them sits within a depression, forming a mountain in a moat.

New Horizons won't finish beaming its Pluto flyby data to Earth until the summer of 2016. Scientists will continue to puzzle through the wealth of information to help them better understand the dwarf planet at the far reaches of the solar system. In the meantime, the spacecraft travels onward to encounter an object in the distant Kuiper Belt.

Salt water flows on Mars

While signs of liquid water on Mars have been spotted over the years, cold temperatures and a thin atmosphere have blocked attempts to directly see such liquid. The September announcement of hydrated salts on the slopes of Mars strengthened the case for flowing water on the Red Planet.

In 2011, scientists spotted strange features known as recurring slope lineae (RSL) on some of the slopes of Mars. The streaks appear during warm weather but vanish in cooler temperatures, leading some scientists to speculate that flowing water played a role in their formation. Scientists found hydrated salts, which form from liquid water, in regions where RSL lie, creating what the lead scientist called a "smoking gun" for flowing water.

Earth's long-lost cousin

In July, NASA's Kepler space telescope spotted the most Earth-like world found to date. Kepler-452b, is a rocky world just bigger than Earth that orbits its sunlike star at about the same distance as Earth does its sun. The world is about 60 percent wider than Earth, which gives Kepler-452b a good chance of being rocky. Its parent star is older than the sun, which means Kepler-452b has had about 1.5 billion years longer than Earth to develop life. [Earth's Cousin Explained: Kepler-452b (Infographic)]

Other rocky worlds with similar sizes to the Earth have been found, but those orbited M-dwarfs, stars that are dimmer than the sun. Kepler-452b is the first Earth-size world found around a sunlike star in an orbit that would allow the planet to keep water at its surface.

Dawn at Ceres

Pluto wasn't the only dwarf planet to receive a visitor. In March, NASA's Dawn mission visited Ceres, an icy world in the asteroid belt. Far from being just another rock, Ceres is an ice-covered world with bright spots that scientists think may be made of either ice or salts. These bright spots form patches of localized atmosphere on the dwarf planet.

Ceres has only one mountain, which mission team members have dubbed "The Pyramid." The crag boasts a flat top and streaked sides, as though material is running down.

Dawn will continue studying Ceres until June 2016.

A yearlong space mission

American astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko reached the halfway point of their yearlong mission on board the International Space Station in September, one of the longest astronaut trips to date.

Health studies on the mission will help doctors better understand the effects of extended space travel on the body. If Kelly remains until March 3, 2016, his trip will be the longest taken by a NASA astronaut.

Rare supermoon eclipse

A rare supermoon lunar eclipse in September delighted skygazers around the world. Although eclipses occur frequently, this year's treat occurred while the moon appeared slightly larger in the sky than usual.

Supermoon appearances result from the satellite's annual trip around the moon; as it reaches its closest point in orbit, it appears about 14 percent larger and 30 percent brighter than it does at its most distant point. The result, in this case, was a spectacular eclipse that drew skywatchers around the world outside. [See more photos of the rare supermoon lunar eclipse]

Only five supermoon eclipses occurred in the 20th century. The next one won't occur until 2033.

A global ocean

Saturn's moon Enceladus startled scientists when NASA's Cassini mission revealed geysers bursting from the moon's southern pole in 2005. After almost 10 years, scientists confirmed that the source of the water was not a local pool beneath the pole but rather a global ocean on Enceladus, which somehow managed to stay liquid over the 4.5-billion-year life of the solar system. Tidal interactions with Saturn are the most likely explanation.

A past Cassini flyby revealed the presence of hydrothermal activity within that ocean, making Enceladus a prime candidate for a site where life evolved beyond Earth. This and other discoveries have lead to discussion about a mission to this Saturn moon.

Happy birthday Hubble!

In April, NASA's Hubble Space Telescope celebrated the 25th anniversary of its 1990 launch. The mission, which initially suffered from blurred vision due to a problem with its mirror at launch, has gone on to capture the public's imagination, with its incredible pictures of distant astronomical events and features, and of worlds within the solar system.

The mission is slated to continue until at least 2020.

Ancient galaxy discovered

A galaxy from the first 600 million years after the Big Bang was spotted in August, the most distant galaxy observed to date. EGSY8p7 lies about 13.2 billion light-years from Earth. Because of the correlation between space and time, scientists are able to observe the galaxy as it appeared 600 million years ago.

Further studies will allow researchers to better understand how the universe evolved in its youth.

NASA and "The Martian"

When the Hollywood blockbuster "The Martian" hit theaters in October, the film industry wasn't the only group to promote the picture. NASA worked with the movie as a way to garner support for its own Mars exploration plans, which aim to send astronauts to the Red Planet in the 2030s.

In the movie, an isolated astronaut struggles with stretching food, water and oxygen supplies to survive on the Red Planet until he can be rescued. Mars colonies of the future will face similar survival needs, though hopefully without the same urgency.

Both the movie and the Andy Weir novel it was based on were heralded for the attention to scientific detail shown by the writers.